The present invention relates to thermal mass flow sensors, and more particularly to thermal mass flow sensors which provide a uniform heater density over a specified length of a flow sensor tube.
The mass flow rate of a fluid is known to be proportional to the amount of heat required to maintain a constant temperature increase in a fluid as it flows through a laminar flow channel. The heating power per unit length of a heated flow passageway in a thermal flow sensor is referred to herein as the heater density.
Various known thermal mass flow sensors employ a uniformly heated flow sensor tube with thermal sensors at various locations along the length of the heated tube. Still others include thermal grounding elements at the ends of the heated tube in order to hold the fluid at ambient temperature at least at those points. Depending on the location of the thermal grounding elements, this can have the counterproductive effect of dissipating much of the heat intended to be transferred into the fluid, thereby vastly reducing the sensitivity of the flow meter.
Blackett, Henry and Rideal1 disclose a flow sensor tube which is uniformly heated along its length so as to establish a constant heater density. The ends of the sensor tube are thermally grounded to a surrounding a case which grips the tube at the sensor tube outer ends, forming a pair of thermal clamps which dissipate heat from the sensor tube. A zero-flow temperature distribution is established which reaches a maximum halfway through the tube and a minimum (i.e., ambient temperature) at the two clamped ends of the tube. When fluid flows through the tube, the fluid absorbs heat as it approaches the maximum temperature distribution point halfway through the tube and releases heat as it leaves the point of maximum temperature distribution. Temperature sensors (thermocouples) are placed at specific points along the sensor tube between the thermal clamps, preferably symmetrically spaced from the center of the tube and within the uniformly heated portion of the tube, to detect a difference in the temperature of the flowing fluid at the sensor locations. The temperature difference is proportional to the mass flow rate of the fluid.
1Blackett et al., xe2x80x9cA Flow Method For Comparing The Specific Heats of Gasesxe2x80x9d, Proc. R. Soc. London, A 126, pp. 319-354 (1930). 
Blackett, Henry and Rideal thus solved the general problem of identifying the optimum location of a pair of point thermal sensors on any uniformly heated flow tube with thermal end clamps. However, because they did not consider a flow tube having thermal clamps outside of the uniformly heated portion of the tube, they did not realize the significance or the extent of the heat transferred at the ends of the heated portion of the tube.
It is apparent from the variety of designs for flow sensors that a complete understanding of the heat transfer between the flowing fluid and the flow sensor tube has not heretofore been reached or shown in the prior art, and therefore the optimum placement of thermal sensors and thermal grounding elements in order to obtain highest temperature sensitivity and fastest response time has not been determined.
U.S. Pat. No. 5,693,880 to Maginnis, Jr., assigned to the assignee of the present invention, discloses a mass flow meter that is relatively insensitive to the position of the thermal grounding elements on the sensor tube and to temperature fluctuations inside the sensor tube. The flow sensor of Maginnis, Jr. includes a split heater coil which is non-uniformly wound around the sensor tube, to provide a non-uniform heater density over the heated portion of the sensor tube. That design used spatially extended heater/sensor elements of non-uniform heater density to achieve an approximately triangular temperature profile along the sensor tube, to reduce sensor noise as well as sensitivity of the flow sensor to errors in the position of the thermal clamps.
The invention advances the state of the art beyond the Blackett, Henry and Rideal configuration by replacing their uniform heater which extends all the way to the thermal clamps with a centrally located uniform heater that does not extend all the way to the thermal clamps. The abrupt drop in heater density at the ends of the uniform heater region causes maximum flow-induced heat transfer between the flow sensor tube and the fluid therein precisely at those ends. The analysis of Blackett, Henry and Rideal of the optimum location for point temperature sensors is therefore not correct for a uniform heater that does not extend through the full length of the sensor tube between thermal clamps.
According to one aspect of the invention, there is provided a thermal mass flow sensor, comprising:
a) a flow sensor tube defining an inlet, an outlet, and an interior channel for fluid flow between the inlet and outlet;
b) a heating element in thermal communication with a portion of the flow sensor tube for heating a fluid flowing therethrough;
c) a pair of thermal sensors in thermal communication with the flow sensor tube, each sensor located at a respective end of the heating element on the flow sensor tube, for sensing the temperature of the fluid flowing through the flow sensor tube and providing a signal representative of the fluid temperature at the respective sensor locations; and
d) a pair of thermal grounding members on the flow sensor tube beyond the ends of the heating element, for establishing respective inlet and outlet reference temperatures for the fluid flowing through the flow sensor tube at the respective locations of the thermal grounding members.
The heating element preferably comprises a heating coil which is substantially uniformly wound around the flow sensor tube and establishes a substantially uniform heater density over the portion of the flow sensor tube heated by the heating element.
The length of the heating coil is determined by the requirement of efficient heating of the fluid in the absence of flow. This is well known in the prior art. The full length of the sensor tube, as measured between the thermal clamp locations, is preferably approximately equal to twice the length of the portion of the sensor tube directly beneath the heating coil, and the heating coil is preferably substantially centered on the flow sensor tube.
The length of the thermal sensors is short relative to the length of the heated portion of the sensor tube. In one preferred embodiment, the thernmal sensors comprise matched thermistors. In an alternate embodiment, they comprise heat flux sensors.
These and other objects and advantages of the invention will in part be obvious and will in part appear hereinafter. The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, the scope of which will be indicated in the claims.